A Myrmyxicus is a truly terrifying sight. Its greasy green body is eel-like, measuring nearly 25 feet in length. A tall row of spines runs down its back, and the tip of its tail ends in a slavering lamprey mouth. The myrmyxicus has a powerfully muscled humanoid torso with four arms arranged symmetrically around the chest which hold scythes. Six octopoid tentacles are arranged in a similar manner around the creature's waist, where its torso turns into its long eel-like tail. The head of a myrmyxicus resembles that of a prehistoric sea reptile, with a fanged snout and tiny black eyes, but with the addition of three pairs of curling horns like those of a ram.

The devastating and terrible Myrmyxicuses are among the most powerful of the Tanar'ri. They are feared and respected by most anything they encounter. They rule vast regions of the Abyssal oceans from floating citadels made of millions of tons of fish skeletons and razor-sharp shells and coral. They are their own masters, and they never serve any other creature for long. Many myrmyxicuses spend ages looking for ways to transport their floating citadels into the seas of other Abyssal layers or even other planes to expand their enslaved empires.

Unlike most powerful tanar'ri, the Myrmyxicus does not have a huge array of spell-like powers to rely upon. Not that it needs such powers; in combat, few creatures can match the horrible fury of an enraged myrmyxicus. It spreads its attacks out among any creatures it can affect, focusing its attacks on a single target only in the rare case when it meets a creature that poses a challenge.

Once per minute, a Myrmyxicus can exhale a 40-foot cone of unholy black vapor that nauseates any creature and additionally does damage to good-aligned creatures.

A Myrmyxicus can also constrict creatures, and grab and grapple with them.

It has some spell-like abilities (though as said before, not quite as many as Tanar'ri usually do), and once per day, it can summon 2 Skulvyns or a Hezrou.

More notably, the Myrmyxicus has the ability to infuse any masterwork scythe it touches with unholy magical energies, transforming it into an unholy scythe for as long as the demon remains in physical contact with the scythe. A myrmyxicus may infuse up to four scythes at a time in this manner; most carry four huge masterwork scythes with them for this purpose.

Finally, the thick, tarlike blood of a myrmyxicus sprays upon any creature that deals damage to the demon with a melee attack using a piercing or slashing weapon or natural weapon. Normal creatures become nauseated by this, while good-aligned creatures take damage.